Changes of surface dynamic parameters under two vegetation coverages

Changes in four main surface parameters, resulting from comparisons from potential to current vegetation distributions, are shown in Fig.9.19: albedo, surface roughness, leaf area index and total vegetation fractional coverage. In those areas where the natural vegetation (mainly forests) have been turned into farmland or where grasslands have been turned into semi-desert or desert (the green area shown in Fig.9.18), the significant decrease in surface roughness and leaf area index is shown in blue (Fig.9.19a,b); increase in albedo is shown in red (Fig.9.19c). The total fractional vegetation coverage is higher in the farmland area than in natural forests (the red area in Fig.9.19d), but is lower in the semi-desert and desert areas in comparison with grassland and in areas of mixed forests in comparison with evergreen broadleaf forests (the blue area in Fig.9.19d). Changes in these surface parameters would certainly modify the exchanges of energy and water between the land surface and the atmosphere and result in the changes in atmospheric circulation as shown in the next section.

9.6.4 Changes of the East Asia monsoon by human-induced land-cover changes

To examine the potential modification of the East Asian monsoon system resulting from land-cover changes, the analyses focus here on the changes in the monsoon circulation and related surface climate. Fig.9.20a presents the mean changes of vector wind and geopotential height in the lower atmosphere in summer. The weakening of the monsoon depression is shown by the positive anomalies in the region to the south of 30°N and the weakening of summer monsoon are shown by the northerly anomalous flow. There is a negative departure in geopotential height over the northern part of the domain, representing the development of a low-pressure system over there, which brings about the anomalous north-west flow.

The changes in mean meridional circulation and zonal circulation are shown in Fig.9.20b, c with major characteristics of the enhancement of descending motion flows over 35-40°N and 100-115°E respectively, which would prevent the development of the summer monsoon circulation. Both these two northerly anomalous flows and the enhancement of descending motion over East Asia would prevent the northward transport of moisture, and the development of convective activities, resulting in more dry conditions in the atmosphere over most of the domain.

(a) Vector wind and geopotential height

(a) Vector wind and geopotential height

20n 25n 30x 35n 40n 45n son i«e lost noE use 120E 125e iaoE 135e hoe

Fig. 9.20 Changes of summer monsoon circulation over East Asia under two vegetation covers (current minus potential) during summer (JJA). (a) vector wind and 850 hPa geopotential height in m (blue: north wind, red: south wind, pink: positive, green: negative); (b) mean meridional circulation along 100-120°E; (c) mean zonal circulation along 25-40°N

20n 25n 30x 35n 40n 45n son i«e lost noE use 120E 125e iaoE 135e hoe

Fig. 9.20 Changes of summer monsoon circulation over East Asia under two vegetation covers (current minus potential) during summer (JJA). (a) vector wind and 850 hPa geopotential height in m (blue: north wind, red: south wind, pink: positive, green: negative); (b) mean meridional circulation along 100-120°E; (c) mean zonal circulation along 25-40°N

Fig.9.21 presents the changes in surface climate related to summer monsoon changes. All components of the surface water cycle, including precipitation (a), runoff (b) and soil moisture (c) are reduced over most of the region. It indicates the weakening of the water cycle through the deterioration of the natural vegetation. There are no significant changes in surface temperature except for a relative warming of an area in the northern China plain (Fig.9.21d), mainly related to the significant reduction in surface evaporation.

In contrast, the winter monsoon over East Asia becomes stronger with the deterioration in the natural vegetation cover, as shown by the strong anomalous northerly flow in the differential fields of vector wind and geo-potential height at 850 hPa (Fig.9.22a). This circulation pattern would bring dry and cold air masses from inland down to all regions of East Asia, resulting in changes in the surface climate, for example the reduction of atmospheric humidity (Fig.9.22b) and precipitation (Fig.9.22c), mostly in the southern part of the region, and cold temperatures over almost the whole region (Fig.9.22d).

(a) Precipitation <b) Runoff

(a) Precipitation <b) Runoff

(c) Soil water (d) Ground temperature
Fig. 9.21 Changes of surface climate over East Asia under two vegetation covers (current minus potential). (a) Precipitation (mm d-1); (b) runoff (mm d-1); (c) soil moisture (mm d-1); (d) ground temperature (K)

(a) Vector wind and geopotential height (to Specific humidity

(a) Vector wind and geopotential height (to Specific humidity

(o) Precipitation (d) Ground temperature

Fig. 9.22 Changes of winter (December, January, February) monsoon circulation and related surface climate. (a) Vector wind and geopotential height at 850 hPa in m (blue: north wind, red: south wind, pink: positive, green: negative); (b) surface humidity (g kg-1); (c) precipitation (mm d-1); (d) ground temperature (K)

Fig. 9.22 Changes of winter (December, January, February) monsoon circulation and related surface climate. (a) Vector wind and geopotential height at 850 hPa in m (blue: north wind, red: south wind, pink: positive, green: negative); (b) surface humidity (g kg-1); (c) precipitation (mm d-1); (d) ground temperature (K)

9.6.5 Conclusions

According to the above analysis, human-induced land-cover changes have modified the monsoon circulation by the weakening of the summer monsoon and the enhancement of the winter monsoon over East Asia, which result in related changes in the surface climate over the region. The conclusions derived from the numerical experiments have been proven by observations. For example, the time evolution of the aridity index over East China since 1880, showing a significant trend in aridification during the last 120 years, with a 36-year period of oscillation (the Brucker period) superimposed on it (Fu 1994). Since the moisture condition over East China is mainly related to the intensity of the summer monsoon, it is a reflection of the weakening of the summer monsoon during that period. On a longer time scale, a 25 000-year lake level dataset for the Daihai Lake in Inner-Mongolia shows a significant reduction in its level, beginning about 3000 years ago (Wang, S. M., pers. comn.). There is also an indication of an aridification trend over Northern China and therefore the weakening of the summer monsoon since then. Over a much longer period, the proxy datasets of winter and summer monsoon indices from loess deposition show an overall dry trend since 2.5 Myears ago.

It seems that the deterioration in natural vegetation due to development of human society is one of the most important anthropogenic factors superimposed on the natural variability of the monsoon system.

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